Power transistors are often used in applications which require that they be operated as power switches, i.e. either fully saturated or off. Such an application may be, for example, an inverter wherein a pair of series-connected transistors are coupled across first and second voltages wherein the transistors are alternately operated to produce an alternating current and voltage at a junction therebetween for energizing a load. In such an application, it is critical that the switches, when on, be operated at an appropriate saturation level to minimize the turn on and turn off times and to minimize losses in the switches.
Proportional base drive circuits have been devised wherein the base current delivered to the transistor is controlled as a function of the current in the collector thereof. Such circuits utilize a current transformer having a primary winding connected in series with the collector of the transistor and a secondary winding coupled across the base-emitter junction of the transistor. An additional, or drive winding, is also provided on the current transformer core and is magnetically linked to the secondary winding. In operation, a drive pulse is developed in the drive winding by a base drive power supply which turns on the transistor via the secondary winding. This in turn causes current to flow in the collector and hence the primary winding. The current flow in the primary winding causes a voltage to be induced in the secondary winding which in turn develops a regenerative current that continues to operate the transistor in saturation even after the drive pulse is removed. The proportional base drive circuit thereby operates the transistor as a constant gain current amplifier. Such a proportional base drive circuit is disclosed in a paper entitled "A New Universal Proportional Drive Technique for a High Voltage Switching Transistor" by Ralph Carpenter of ELDEC Corporation, Lynwood, Wash., published in the Proceedings of Powercon 8, D-2 pp. 1-15, 1981.
However, it has been found that the core of the current transformer must be periodically "reset" to eliminate the deleterious effects arising from hysteresis of the core. This reset function is accomplished by the base drive power supply which develops a reset pulse of proper polarity in the drive winding. This necessity to reset the current transformer core unduly complicates the design of the base drive power supply because the timing of the reset pulse must be accurately controlled and the power supply must be capable of providing a pulse of significant amplitude.
The above type of design is also undesirable in those applications where two or more switches are to be controlled using a base drive proportional scheme. In this case, a primary winding must be connected in series with the collector of each transistor, in turn significantly increasing the size and weight of the circuit in which the transistors are used and increasing the number of separate conductors which are needed. This is particularly disadvantageous in those applications which utilize power switches colloquially termed "hockey puck" transistors wherein the main power electrodes of the device comprise upper and lower metal faces thereof with a separate connector provided for the base electrode so that two such transistors can be connected in series simply by pressing the appropriate faces together. In such an application, the extra conductors cause problems in packaging and introduces additional undesirable inductance into the output stage of the circuit.
An alternative type of proportional base drive circuit is disclosed in Kammiller et al U.S. Pat. No. 4,493,017. This proportional base drive circuit includes a single transformer which includes a primary winding which receives base drive signals, first and second secondary windings coupled to the bases of first and second power transistors, respectively, and a regenerative winding coupled in series between a load and the junction between the first and second power transistors. The application of a pulse to the primary winding causes the first power transistor to conduct, in turn causing load current to flow through the regenerative winding to the load. The current flowing through the regenerative winding causes a regenerative current to flow in the first secondary winding to maintain the first power transistor in a staturated state. Subsequently, the second drive transistor is turned on and the first transistor turned off by application of a pulse of opposite polarity to the primary winding, which in turn causes a load current of opposite direction to flow in the regenerative winding. This current induces a voltage and current in the second secondary winding to maintain the second transistor in the saturated state.
Similar types of proportional base drive circuits are disclosed in Maeda et al. U.S. Pat. No. 4,424,556, Farrer et al U.S. Pat. No. 4,319,316, Wellford U.S. Pat. No. 3,588,669 and Great Britain Patent Application No. 2 071 950 A. Further, the last-mentioned Great Britain patent application discloses such a base drive circuit in connection with a series-resonant inverter.
Such types of base drive proportional circuits, however, cannot be easily adapted for use with neutral point clamped inverters. Also, the turn off time of the power switches is not minimized. Nor is there any provision for protection circuitry to disable the switching devices in the event of an overload.